Sec. 5-1] 



RADIA TIOX TliA SSDUCE US 



237 



70 per cent of the radiation at 250 m/u. Quartz transmits ultraviolet 

 light down to 200 m/u and begins to absorb substantially at shorter 

 wavelengths. Ultraviolet radiation and radiation of smaller wave- 

 lengths can also be measured by converting it into visible light by 

 means of a fluorescent screen and measuring the screen brightness 

 with photoelectric devices that are sensitive to visible radiation. 1 



100 



80 



60 



t 40 

 ce 



20 



100 300 500 700 



Wavelength, m/i 



900 



> 1r c; 



<d 



V- o => <" = ~ D >- 



Fig. (5-1)4. Relative sensitivity of different 

 commercial photocathodes (from RCA Tube 

 Manual ; by permission). 



Since the photocathode response varies with the wavelength, the 

 integral magnitude "sensitivity," in amperes per lumen, has only a 

 limited significance. The sensitivity of photoelectric devices can be 

 compared either on the basis of a light source of equal radiant flux 

 at all wavelengths or of a standardized light source with known and 

 reproducible spectra] distribution. For the measurement of the lumi- 

 nous sensitivity, it has become standard practice to use a tungsten 

 lamp operated at a filament temperature of 2870°K. Alternatively, 

 1 hololumen, i.e., the radiant flux of all wavelengths of a tungsten 

 lamp at a color temperature of 2848°K, is used which, evaluated as 



X R. D. Cowen, Bull. Am. Phys. Soc, 22, F-7 (December, 1947; 



